Our program is devoted to answering key questions in biology through the development and application of multinuclear electron-nuclear double resonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) spectroscopies to characterize transition-metal centers vital to human health and disease. Application of these techniques to intermediate states that are trapped by freeze-quench and cryoreduction methods, and are isotopically labeled by outstanding teams of collaborating chemists and biologists, can completely characterize the active-site environment of a metal ion at every stage of an enzyme's catalytic cycle. As augmented by novel kinetic protocols we have developed, these studies reveal in precise details the function of critical metal centers, not only in isolate enzymes but in living cells as well. Dramatic progress during our current grant period has inspired multiple specific aims: (i) Small-molecule activation by metalloenzymes, including Dioxygen-activation by Heme Monooxygenases and Dinitrogen Reduction by Nitrogenase; (ii) Biomimetic Mo and Fe Complexes as Models for Nitrogenase Intermediates and Paradigms for the Jahn-Taller Effect in trigonal symmetry; (iii) Radical Reactions in Metalloenzymes -The Radical SAM Superfamily; (iv) in vivo Speciation of Mn(II) - Protection from Oxidative Stress, Mechanisms of Toxicity, Probe of Physiology; (v) ATPase Co(II)/Mn(II) Transporters; (vi) ENDOR Methodology Development - Hyperfine Signs, Electron-Nuclear-Electron Triple Resonance, Q-band Resonator Development; (vii) Extending the Biological Applications of ENDOR Techniques. Many of these aims reflect longstanding efforts, while projects associated with transition-metal homeostasis that emerged this period (Mn(II) speciation, and Mn(II)/Co(II) transport) reflect a commitment to opening new areas, as explicitly expressed in the final Aim.